D. L. Sprague, N. J. Fedele and L. D. Ryan in a U.S. Pat. No. 4,740,832 issued 26 April 1987, entitled NON-DEDICATED IMAGE MEMORY USING SEPARATE BIT-MAP ORGANIZATIONS FOR LUMINANCE AND CHROMINANCE VARIABLES and assigned to RCA Corporation describe a system for retrieving stored images from video random-access memory (VRAM). A VRAM is a dual-ported memory including a dynamic random-access memory with a random-access read/write port and including a relatively small, auxiliary, static serial memory with a serial output port. The auxiliary memory can upon command have its storage locations loaded in parallel from any row of storage locations in the larger dynamic memory. Thereafter the auxiliary memory has its storage locations scanned by a counter operating as an address generator and is read out in a shift register operation to supply a stream of video data.
In the Sprague-Fedele-Ryan system images are described in terms of luminance and chrominance components, each of which has its own bit-map organization associated therewith in the dynamic memory portion of VRAM. Groups of bits descriptive of the luminance or chrominance of a pixel are stored together in a conformal mapping of the display in a "bit-map-organized" memory as that term is employed in this specification. The luminance components are generally more densely sampled in image field space than the chrominance components are; this is done to conserve image memory, recognizing that visual acuity for chrominance is less than that for luminance. VRAM is "linearly packed"--i.e., the raster scanning of pixel codes is stored in successive rows of the dynamic memory. Rows in dynamic memory do not necessarily have a 1:1 correspondence with scan lines in the ultimate display. A formatter known as a "pixel unwrapper" takes a stream of data supplied to it from the VRAM serial output port and passes it into scan lines of successive pixel codes.
During line trace intervals in the display, VRAM from its output port supplles data from which data the pixel-unwrapper generates a stream of pixel codes describing luminance in real time. During selected line retrace intervals in the display, VRAM supplies data from its serial output port from which data the pixel unwrapper generates two streams of pixel codes describing chrominance in a compressed-in-time and advanced-in-time format. Each stream of chrominance components is supplied to a respective chrominance re-sampling apparatus, each of which re-sampling apparatuses comprises a respective odd-line line-storage memory, a respective even-line line-storage memory and an interpolator. Successive lines of each stream of compressed chrominance data are selcted on an alternating basis for writing into its odd-line or its even-line line-storage memory. These line storage memories act as a rate-buffer to supply samples to their interpolator, which generates samples of the chrominance component with compression removed and with delay to temporally align them with the real-time luminance samples. The luminance samples and the two sets of chrominance samples are converted from digital to analog form and are linearly combined, for generating red, green and blue analog video signals. These analog video signals are amplified and gamma-corrected to provide drive signals for the display apparatus, typically a color kinescope.
The Sprague, Fedele and Ryan interpolator uses a cascade of n basic interpolator blocks and a multiplexer to re-sample each set of supplied chrominance samples 2.sup.n times more densely in both the direction of pixel scan and the direction of line advance. Each basic interpolator block includes three multiplexers, three adders, two clocked unit-delay latches and bit-place shift circuitry.